Field of Invention
This invention relates generally to xe2x80x9cpolyxe2x80x9d bag making machines and, more particularly, to methods and apparatus for controlling the web tension in such bag making machines.
Poly bag making machines are well known. Generally, draw rolls pull a two-ply web of plastic film material from a supply roll. A transverse cutting and sealing bar (hereinafter referred to as a seal bar) is situated after the draw rolls and is mounted for reciprocation to cut and seal the web after each web index movement to form individual bags. The bags are carried to a stacking station which is situated on a stacker conveyer, by means of a rotating vacuum arm or other assembly. Bag machines of the type described are well known. For example, a typical bag making machine of this type is The Polystar 9000 available from Ro-An Industries Corp. of Maspeth, N.Y., U.S.A.
Generally, bag making machines of the type described are powered by a main drive motor that drives a main drive shaft which in turn drives various components of the bag making machine, including the draw rolls, the seal bar, the vacuum arm assembly and the stacker conveyer. A dancer apparatus is situated between the web supply roll and the draw rolls for adjusting and controlling the tension in the moving web as it is drawn by the draw rolls.
Recently, servo drives have been used to drive various components of bag making machines. For example, a servo motor is used to drive the seal roll in the apparatus disclosed in U.S. Pat. No. 5,230,688 to Hatchell et al. Servo motors are used to drive the draw rolls and stacker conveyer components in a bag making machine disclosed in U.S. Pat. No. 5,338,281 to Terranova. The disclosure of both of these patents is incorporated herein in their entirety.
The poly bag making industry is moving to thinner plastic film material for bags to reduce cost. Thinner plastic film material is more difficult to feed through a bag making machine because a lower web tension must be used than in the case of thicker film material in order to minimize material stretch and/or breakage.
In a typical bag making machine the draw rolls pull the web with an intermittent motion. The web supply roll is too large and heavy to permit such intermittent motion, so a dancer roll apparatus is typically installed between the supply roll and the draw rolls to absorb the intermittent motion. However, the dancer apparatus used in current bag making machines has too much weight and therefore too much inertia to effectively handle thin web materials. Thus, when the draw rolls pull the web, excessive tension is developed in the web in lifting the dancer rolls and in pulling the web from the supply roll through the unwind stand and then through the various parts of the machine. The result with thin film is excessive stretching or breakage.
An object of the present invention is to provide new and improved web tensioning devices and methods for poly bag making machines.
Another object of the present invention is to provide new and improved web tensioning devices and methods for poly bag making machines especially for use with thin web materials.
Briefly, these and other objects are attained by providing, in lieu of the conventional dancer apparatus, a tension control system including a servo-driven unwind roll for the web supply roll, a vacuum box situated between the web supply roll and the bag machine draw roll which receives a loop of the film web under a partial vacuum, a servo-driven capstan situated at the inlet side of the vacuum box which engages the film web and a servo-driven draw roll. A servo-controller is programmed to control the motion and position of the unwind roll, the capstan and the draw roll, to thereby independently control the tension in a first run of the film web between the supply roll and the capstan and the tension in a second run of the film web between the point of separation from the capstan and the draw roll.
The invention thus eliminates the conventional dancer apparatus and instead utilizes a new tension control system and structure. For convenience, zone 1 is defined as the run of the web extending from the capstan upstream to the supply roll, and zone 2 is defined as the run of the web extending from the point of web separation from the capstan downstream to the draw rolls. In this invention tension control in zone 1 is separate and independent of tension control in zone 2.
This vacuum box is a chamber through which the fast moving web is passed. A partial vacuum draws the web toward the bottom of the chamber, thus forming the web into a dynamic loop which loop becomes deeper or shallower, although its nominal position is about midway of the total box depth. The depth of the loop is varied by varying the speed or power of the vacuum blower or by varying the opening of a bleed valve.
The new system includes a servo driver or amplifier and servo motor to drive a surface drive roll to unwind the supply roll, a servo driver or amp and servo motor to drive the capstan at the intake side of the vacuum box, a servo driver or amp and servo motor to drive the draw roll, and a depth sensor for the web loop in the vacuum box. All these components are in feedback circuitry with a main motion controller, a programmable logic controller and auxiliary motor drive elements.
The system reduces web tension and variations in web tension in three ways:
a. the conventional dancer apparatus with its weight and inertia is eliminated and thus no mechanical parts are accelerated by tension in the web in the run between the vacuum box and the draw rolls;
b. the supply roll is driven by a servo powered surface drive roll so that the web does not have to provide the force to unwind the film material; and
c. the torque mode capstan pulls the web through the unwind stand and machine infeed rollers but does not have to provide the force to move the supply roll.
The torque mode capstan thus defines and separates the two zones of tension within the system. Web tension in the zone 1 run between the supply roll and the torque mode capstan is directly proportional to the torque setting of the servo motor driving the capstan; and the tension in the zone 2 run of the web from the torque mode capstan to the draw rolls is controlled by the level of vacuum supplied by the vacuum blower. All the servos are controlled by an intelligent motion control system.
The new system may be operated to (a) anticipate an interrupt, for example, as when the draw rolls intermittently stop to permit the stacker conveyor to index forward a completed stack, and/or (b) detect an improper web tension situation in zone 1 or 2, or (c) permit an operator or a software program to alter operation parameters. The motion controller then forecasts a new set of servo settings and directs the servo drivers or amplifiers accordingly, which provide feedback leading to consecutive re-settings until the desired operation is achieved.
In the example where the system is programmed to anticipate an interrupt, when the draw rolls stop to permit the stacker conveyor to index forwardly, the supply roll continues to feed web material. The web entering the vacuum box will therefore tend to develop a deeper loop since the web downstream of the box has stopped. To avoid an excessive web back-up in the vacuum box, the loop depth is, just before the interrupt, re-set to be much shallower. Then, upon interrupt, the web entering the vacuum box will cause the shortened loop to extend to a full loop, thereby maintaining the loop in proper form and order. The constant feedback from the loop depth sensor tells the servo powering the surface drive roll to pay out more or less web to keep the loop in the correct position in the vacuum box.
During a forecast and system alteration in the zone 2 web run, as generally described above, the web tension in the zone 1 run is maintained generally constant by the torque mode capstan. Detailed machine sequences are explained below in the description of the preferred embodiments.